The gamma-glutamyl carboxylase generates carboxylated Glus (Glas) in the Gla domain of vitamin K- dependent (VKD) proteins to render them active in a diverse range of functions that includes hemostasis, calcification, apoptosis and growth control. A single carboxylase modifies all VKD proteins, and this broad physiological impact makes it critical to understand the mechanisms of catalysis and regulation. Naturally occurring carboxylase mutations cause vitamin K-dependent clotting factor deficiency (VKCFD), which is associated with severe bleeding. Mutations have recently also been shown to cause a second disease, pseudoxanthoma elasticum (PXE), where bleeding is mild but calcification is defective. Why carboxylase mutations cause this second disease was unknown, and our studies on a carboxylase mutation that causes PXE indicate a defect in fully modifying the Gla domain of VKD proteins. The observation that carboxylase mutations cause PXE highlights the importance of understanding how carboxylation fully impacts human health. The role of carboxylation in hemostasis is well established; however, the carboxylase is expressed in virtually every tissue and nonhemostatic functions are very poorly understood. These functions are likely to be affected in the many patients treated with Warfarin, which inhibits carboxylation. Our goals in the next funding period are to understand the mechanism of VKD protein carboxylation, determine how carboxylase mutations cause two different diseases, and begin defining the role of carboxylation in different tissues. In Aim 1, we will test a novel hypothesis or how the carboxylase initiates catalysis, and will also determine how two naturally occurring carboxylase mutations cause VKCFD. Aim 2 will determine how undercarboxylation impacts PXE using a novel method we developed to analyze VKD proteins by mass spectrometry. Aim 2 will also use an innovative mouse model to determine whether the carboxylase plays a role in calcification in soft tissue. The carboxylase contains Glas, of unknown function, and we have now identified the Gla domain. In Aim 3, we will use a mutagenesis approach to test the hypothesis that the Gla domain regulates VKD protein carboxylation, and will use FRET to determine if the Gla domain facilitates phospholipid interaction. VKD proteins are carboxylated as part of a secretory process that impacts carboxylation, and Aim 4 will test the role of cellular components in mediating VKD protein trafficking. These studies will provide new fundamental information important for the development of mechanism-based inhibitors that lead to superior anticoagulants, and will also be significant for the production of therapeutic VKD proteins for applications in hemophilia, sepsis and tissue glues.